The orbits of artificial satellites are determined in accordance with application purposes. A so-called stationary orbit satellite such as a communication satellite or a meteorological satellite must be held in a predetermined orbit until the end of its service life. Therefore, if the service life of a stationary orbit satellite comes to an end or the satellite fails, a new satellite cannot be placed in the predetermined orbit unless the old satellite is removed from the predetermined orbit and discarded.
An orbit correcting apparatus approaches the target satellite to be discarded and is coupled thereto. Subsequently, the coupled system is caused to leave the predetermined orbit by the thrust of the orbit correcting apparatus.
In a conventional method of changing the orbit of an artificial satellite by using such an orbit correcting apparatus, as shown in FIG. 1, for example, the orbit correcting apparatus, which is rotated in an original orbit O1 having an orbital radius R at a peripheral velocity V.sub.c in advance, catches and is coupled to a target satellite at a point P1. The orbit correcting apparatus then generates a thrust to allow a coupled system constituted by the orbit correcting apparatus and the target satellite to obtain a velocity increase .DELTA.V. When the velocity increase .DELTA.V is represented by the following equation: EQU (V.sub.c +.DELTA.V.sup.2)=.mu.(2/R-1/a) (1)
where .mu. is a gravity coefficient which is 638,603 km3/s.sup.2 for the earth, the coupled system constituted by the orbit correcting apparatus and the target satellite is placed in an elliptic orbit O2 as a transition orbit having a semi-major axis a. The target satellite is separated from the correcting apparatus in the elliptic orbit O2, and is placed in a new orbit.
FIG. 2 is a view for explaining an orbit changing operation in more detail. An apogee P2 of the transition orbit O2 is higher than the original orbit O1 by approximately ##EQU1##
When the orbit correcting apparatus is to be reused, the apparatus must return to the original orbit O1 by generating a thrust at the point P1 on the orbit O2, at which the orbit O2 is in contact with the original orbit O1, in the opposite direction in the same manner as in the case wherein the velocity increase .DELTA.V is obtained.
In order to prevent the orbit correcting apparatus from crossing the original orbit, the correcting apparatus is caused to generate a thrust to obtain a velocity increase .DELTA.V' at the apogee P2, and the coupled system is placed in an orbit O3, as shown in, e.g., FIG. 2. The target satellite is rotated in the orbit O3, and passes through a perigee P3 which is separated from the perigee P1. That is, the target satellite acquires the new orbit O3 which does not cross the original orbit.
When the conventional orbit correcting apparatus is to be reused, the coupling of the correcting apparatus with the target satellite is released at, e.g., the point P2. The orbit correcting apparatus then generates a thrust in the opposite direction to return to the orbit O2, and generates a thrust again at the point P1 to return to the orbit O1. The orbit correcting apparatus is thus rotated and reused in the orbit O1.
As described above, when the conventional orbit correcting apparatus is to be reused, the apparatus must leave an original orbit to discard a target satellite in a new orbit and return to the original orbit by using a thrust force in the opposite direction. At this time, since a velocity increase required for the orbit correction is substantially the same as that used to obtain the new orbit, a large amount of thrust energy is required. For this reason, the orbit correcting apparatus is discarded together with the target satellite and is not reused in spite of the fact that reuse of the apparatus is desired.
In addition, even if the orbit correcting apparatus needs to be discarded together with the target satellite, it need not be moved to the same orbit as that of the target satellite. In practice, however, they must be temporarily moved together to the same orbit. This is wasteful in terms of energy.